…-tag CI job

- Move UpdateChecker.h/.cpp from Generals/ and GeneralsMD/ to Core/GameEngine/
  as shared INTERFACE source, eliminating duplication between the two games
- Fix git_watcher.cmake: use 'describe --exact-match --tags' instead of
  'describe --tags' so GitTag is only set on exact tagged commits, never on
  dev builds that have a release tag somewhere in history
- Add create-tag CI job to release.yml that runs BEFORE build jobs, ensuring
  the tag exists in the repository when binaries are compiled (GitTag is set)
- Remove CURL link entries from both GameEngineDevice CMakeLists (now
  propagated transitively via corei_gameengine_public INTERFACE)

Moving the dry_run guard from the inner step to the job level so the
entire job is skipped (status: skipped) rather than running and doing
nothing. Build jobs treat a skipped dependency as success and proceed
normally. No tag should ever be created or pushed in a dry run.

…g is set

GENERALS_FORCE_UPDATE_CHECK=1 was bypassing the guard in start() but not
the second GitTag[0] == '\0' check inside threadFunc, causing the thread
to return early and never set s_hasUpdate. In force mode, treat any
non-empty tag from the GitHub API response as 'update available'.

Add cleanup-tag-on-failure job that runs when create-tag succeeded but
any downstream job (builds or prepare-release) failed. This prevents
the 'tag already exists' error on retry with the same version.

Condition: failure() && needs.create-tag.result == 'success'
- Skipped on dry_run (create-tag is skipped, result != 'success')
- Skipped when everything succeeds (failure() is false)
- Runs only when tag was pushed but builds/release preparation failed

SDL3/SDL.h was included directly in UpdateChecker.h for the private
static member types (SDL_Thread*, SDL_AtomicInt). This caused build
failures on Linux CI and macOS where MainMenu.cpp (part of g_gameengine)
includes UpdateChecker.h but g_gameengine does not have SDL3 on its
direct include path -- SDL3 only enters via g_gameenginedevice.

Fix: convert class static members and threadFunc to file-scoped statics
in UpdateChecker.cpp. The header now exposes only the public interface
(start, poll, getReleasesUrl) with no SDL3 types, following pimpl-lite
pattern for static-only utility classes.

…ATE_CHECK builds

MainMenu.cpp includes <SDL3/SDL.h> directly to call SDL_OpenURL. On macOS
the build passed because vcpkg adds SDL3 headers globally as an isystem
path. On Linux CI (linux64-deploy), SDL3 comes from FetchContent and its
include directories are only propagated to targets that explicitly depend
on SDL3::SDL3 -- g_gameengine and z_gameengine did not.

Fix: add SDL3::SDL3 as an INTERFACE dependency of corei_gameengine_public
inside the SAGE_UPDATE_CHECK block. Since SAGE_UPDATE_CHECK is only ON
when SAGE_USE_SDL3 is also ON, SDL3::SDL3 always exists at this point.
The include dirs propagate transitively to all gameengine consumers.